1. Field of the Invention
The present invention relates to a technique capable of easily eliminating or reducing uneven density.
2. Description of the Related Art
An image forming section of a conventional image processing apparatus includes a plurality of modules, such as a photosensitive drum, a charging device, an exposure device, a developing device, and a transfer device. In the above-described image forming section, the charging device uniformly charges the photosensitive drum while the photosensitive drum is rotating.
Next, the exposure device selectively exposes a charged photosensitive drum surface to form an electrostatic latent image on the photosensitive drum. Then, the developing device develops the electrostatic latent image formed on the photosensitive drum into a visible toner image. The transfer device transfers the obtained toner image onto a recording material.
In the above-described image forming section, uneven density (banding) may appear periodically on the photosensitive drum in a rotational direction thereof, more specifically, in a sub scanning direction. Many of the above-described modules include rotational members, such as drums, rollers, sleeves, and screws, which rotate continuously during their operations.
In general, a motor serving as a rotational power source of each module has an error in its rotational behavior. The rotational speed of each motor is variable depending on the rotational angle of a motor shaft. Therefore, for example, the rotational speed of the photosensitive drum varies periodically in accordance with the rotation of a driving motor.
According to the periodic variation in the rotational speed of the photosensitive drum, the quantity of exposure per unit area changes periodically and periodic uneven density appears on an output image. Further, the accuracy of a gear that transmits motor driving power to a rotary member becomes a cause of the uneven density.
Further, if a rotational shaft of a rotary member has a certain amount of eccentricity, for example, when the photosensitive drum has a certain amount of eccentricity, the distance between the photosensitive drum and the developing device changes periodically. Therefore, a significant amount of periodic uneven density appears due to a change in developing efficiency corresponding to the distance.
Further, the AC voltage applied to the charging and developing devices becomes a cause that may induce the periodic uneven density because the developing amount changes similarly. As described above, a general image forming section includes various modules that may induce the periodic uneven density.
Further, in a case where a device includes a plurality of modules each having a unique periodicity, a significant amount of interference may occur between two or more different frequencies and a composite periodic uneven density may appear.
To solve the above-described problem, as discussed in Japanese Patent Application Laid-Open No. 10-20579, there is a conventional technique capable of adjusting a turn-on period of an exposure device to correct the density based on a generated density variation detected by a density detection device.
Further, the image forming section can include a scanner capable of reading a final image (i.e., a test image) to detect a density distribution. The detected density distribution can be used to correct the periodic uneven density.
For example, as discussed in Japanese Patent Application Laid-Open No. 11-112810, a scanner performs a scanning operation to read a test image and detect density data in a main or sub scanning direction. Then, pixel data correction is performed using correction data generated based on the detected density data. According to any one of the above-described conventional techniques, the periodic uneven density can be reduced by performing correction having inverse characteristics in such a way as to cancel or reduce the generated density characteristics.
According to the technique discussed in Japanese Patent Application Laid-Open No. 10-20579, it is required to measure all information about periodicity, waveform, phase, and amplitude and determine correction parameters. Therefore, it is required to detect the density of an image at intervals sufficiently smaller than the periodicity of the image forming section.
Therefore, the density detection device is required to have highly accurate resolution and response. Further, to equalize the phases of a plurality of periodicities generated by respective modules, it is required to adjust the design features (e.g., an outer diameter of each module) to be the same or in an integer multiple relationship. Thus, the degrees of freedom in the design stage are reduced.
Further, according to the technique discussed in Japanese Patent Application Laid-Open No. 11-112810, it is similarly required to measure information about all of periodicity, waveform, phase, and amplitude and determine correction parameter. Thus, a large-scale device, such as the above-described scanner, is required.
Further, it is required to perform correction at intervals equivalent to an image formed on a transfer member or a paper. Therefore, the rotational periodicity of each module is required to be an integer fraction of the image. If the rotational periodicity of a module is not an integer fraction of the image, the phase of the module causes a stepwise deviation in each output. Therefore, a correction amount calculated based on an output of the scanner cannot be effectively used in the correction.
Further, each of the above-described techniques requires a memory that stores measurement data and a synchronization system that synchronizes measurement timing with correction timing. Accordingly, an apparatus having a large-scale configuration is required and it is difficult to realize a low-cost apparatus.